1. Field of the Invention
The present invention relates to a variable linearity coil which is used in a monitor of a personal computer, or the like.
2. Description of the Related Art
As is well known, the linearity coil is interposed for connection to a horizontal deflection circuit of a monitor of a television receiver set, personal computer, or the like for the purpose of correcting distortions on a monitor screen. In the television receiver set, or the like, the frequency of the electric current to flow or to be charged in the linearity coil is constant at 15.75 kHz or 33.75 kHz. Therefore, the correction of the distortions was possible with a linearity coil of a constant or predetermined DC (direct current) magnetic field. In the monitor of the personal computer, on the other hand, the range of the frequency of the electric current to be charged is as wide as 15 kHZ to 120 kHz. Therefore, there occurs a difference in the amount of correction between the time when the frequency is low and the time when the frequency is high. It follows that an appropriate correction of distortions cannot be made in a predetermined DC magnetic field.
As a solution, there has conventionally been proposed the following variable linearity coil as shown in FIG. 7. Namely, a magnetic core b which is made by winding therearound a coil "a" and a magnetic core d which is made by winding therearound a magnetic field adjusting coil c for adjusting the bias magnetic field are placed one on top of the other in a vertical posture (i.e., with an axis of winding the coil extending in the up and down direction) together with permanent magnets e.sub.1, e.sub.2. The assembly thus obtained is placed on top of a base g which has embedded therein lead terminals f. The lead terminals f are connected to respective terminal ends of the coil "a" and the magnetic field adjusting coil c.
In the above-described conventional variable linearity coil, the magnetic cores b and d are both of a type of open magnetic field path. Therefore, the magnetic fields of the above-described coil "a" and of the magnetic field adjusting coil c will be generated in the vertical direction. The magnetic fields thus generated are likely to give effect on a cathode ray tube which is disposed near the coils "a" and c. Further, an axial line of each of the above-described coil "a" and of the magnetic field adjusting coil c is in parallel with the lead terminals f which are embedded into the base g. Therefore, when the linearity coil is surface-mounted on a printed-circuit board (not illustrated) and sawtooth wave electric current is charged to the coil "a", the magnetic core b is subject to extension and contraction in the vertical direction, thereby giving rise to magnetostriction vibrations to the magnetic core b. These vibrations are transmitted to the printed-circuit board to thereby generate beat notes through resonance. Further, these vibrations also cause vibrations of other neighboring component parts which are surface-mounted on the printed-circuit board, with the result that their reliability is impaired.
In view of the above-described problems with the conventional linearity coil, the present invention has an object of providing a variable linearity coil in which the magnetostriction vibrations of the magnetic core to be transmitted to the printed-circuit board on which the variable linearity coil is surface-mounted can be reduced to the smallest extent possible and in which the occurrence of beat notes due to resonance is prevented to the best extent possible, whereby the reliability of other neighboring component parts which are surface-mounted on the printed-circuit board is improved.